Type-1 adenylyl cyclase (AC-1) is an enzyme located in the brain that has been implicated as playing an important role in learning and memory. A study in the rat showed that the gene for AC-1 is expressed in several groups of neurons in the hippocampal formation, a substructure of the brain that is clearly involved in Alzheimer's disease and other neurological disorders associated with dementia. In this study we tested the hypothesis that AC-1 is located in the hippocampus of primates, and particularly in nerve pathways of the hippocampus, where damage is associated with impairment of long-term potentiation, a neurophysiological phenomenon that is believed to mediate learning at the level of the nerve cell. The study showed that, indeed, AC-1 was readily demonstrated in the hippocampal formation of the primate. Furthermore it was most prominent in the mossy fiber pathway, a substructure of the hippocampal formation most clearly implicated in long-term potentiation. FUNDING NIH grants LM/OD 06243 and RR00166.